In a common approach to channel-dependent scheduling and link adaptation, a base station schedules downlink transmissions to mobile terminals based on which mobile terminal experiences favorable channel conditions. The base station also adjusts one or more downlink transmission parameters (e.g., modulation scheme and channel-coding rate) responsive to those channel conditions. When used in conjunction with one another, as done for High Speed Data Packet Access (HSDPA) services in Wideband CDMA, these technologies greatly enhance downlink performance in terms of higher data rates.
A mobile terminal supports channel-dependent scheduling and link adaptation at the base station by sending channel quality information as feedback to the base station. Channel quality feedback information may indirectly indicate the channel quality as the data rates (i.e., transport block sizes), modulation schemes, and coding rates that can be received by the mobile terminal at a pre-determined reliability or other performance metric. Typically, the mobile terminal derives channel quality feedback information by measuring the noise and interference currently present at the mobile terminal (e.g., as a Signal-to-Interference-Plus-Noise ratio, SINR), and selecting a value for the channel quality feedback information indicative of that measurement. The base station then performs channel-dependent scheduling and link adaptation on the assumption that the noise and interference present at the mobile terminal has not substantially changed since the mobile terminal measured it.
In circuit switched systems, this assumption is often justified because interference at the mobile terminal is approximately continuous (i.e., transmissions to other mobile terminals, such as those in neighboring cells, are continuous). Because interference is continuous, the base station can effectively base link adaptation on the assumption that interference has not significantly changed between the time when the mobile terminal measured it and the time when the corresponding channel quality feedback information is actually used by the base station. Moreover, the mobile terminal can reduce potential error in interference measurements due to random noise by averaging them over a period of time.
In packet-switched systems, however, interference at the mobile terminal may be discontinuous due to the bursty nature of packet data transmissions to other mobile terminals. This can degrade channel quality feedback information provided by the mobile terminal if the time required to average out random noise in the interference measurements is greater than the time the interference is stationary. Of course, even if this is not the case, the channel quality feedback information used by the base station for scheduling and link adaptation may no longer be reliable simply because interference at the mobile terminal has changed since the time the mobile terminal measured it.
Regardless of the specific reason for its unreliability, if the channel quality feedback information used for scheduling and link adaptation indicates there is less interference than actually present at the mobile terminal at the time of transmission, errors in the transmission will likely occur. Conversely, if the channel quality feedback information indicates there is more interference than actually present, the transmission will be at a lower data rate than that actually possible.